Maker of Acoustic Horns since 2001

 
 
 
 

Azura Horn System

'The Voice of the Lounge'

This is a fully horn loaded system that will run off a single SET amp (apart from the sub). You can improvise around it, but I would consider the bass horn to be fundamental to the sound.

The 50 Hz bass horn has few compromises. Perhaps not quite for the lounge room but the footprint is reasonable (110 cm wide at the mouth x 120 cm depth). The L-shape allows it to be half wavelength (170 cm long). The horn mouth perimeter is 1/2 wavelength at 50 Hz and is proportioned to near the Golden Mean. (106 x 66 cm). The large deep horn mouth presents an attractive interior space to the room.

I use Jensen A-12 Field Coil drivers on the bass horns, but have tried many interesting drivers with success from 15" Vitavox K15/40 to 8" Lowthers.

My favourite drivers on the AH-340 horns are JBL2470 or LM555. Look for a driver that can sustain a low 1st order cross over point of 350 Hz. Atlas PD-5VH is a good candidate. The super cheap JBL Selenium D250-X might be worth a try. AH-160 or 204 horns are perhaps better for this, but I like the 340 horns for their proportion and focused sound.

I came up with a variant of jc's 'subtractive' cross over - the iixo or Impedance Inverter cross over that is almost ridiculously minimalist, and seems to be original.

Fostex T900A tweeters on AH-1100 horns come in at 6K Hz. Deep bass is provided by a 3.4m tapped horn.

The horns integrate well at short distance and will maintain an image when standing a metre in front of the centre spot. They will sustain a wide sound stage, I have them 3 m apart. They retain their qualities at low volume. They are good domestic horns.

.Azura Tapped Horn

Deep bass is provided by a 3.4m tapped horn.

Outside dimensions of cabinet:350 x 325 x 1700mm. Effective length 3.4m.
Versions for 10" or 12"/ 15" drivers available as plans.

The tapped horn sound integrates well with the bass horns. Plans are available in versions suitable for 10" and 15" drivers

 

iixo  Impedance Inverter Crossover 

The iixo is nothing more than a series Low pass inductor with a secondary wound on top, connected to the upper driver. This produces a High Pass which is the inverse of the LP output. The basic idea is below. Drivers should be modeled with VC inductance and DCR as this will change things. The step down ratio is chosen to give the required attenuation. 

You need drivers that can survive going below the horn cut off. Note: with a first order filter the driver excursion will continue to increase below the F-3dB point.

The iixo is not as perfect as it looks but it sounds very natural and is easy to try out.  Note: It requires a step down ratio to work. If you want to wind a secondary on top of an existing inductor and don't know the turns you can use  the ratio of the square root of the inductances.

Notes:

Jensen made TA-4165 FC drivers for WE, and perhaps early A-12s share a lot in common with them. They look identical.  http://www.silbatoneacoustics.com/art/1-2p.pdf

Fostex T900A mounted on 1100 Hz horns give crisp percussion and natural projection. The horn gives a little extra sensitivity,  when using a single amplifier. I machined a ring to fit into the T900A horn mouth to match the 1" horn throat. 

The driver enclosures are 35 Litre. I tried the enclosures open and closed and found imaging is better closed. Try both as some prefer the sound quality with them open. HornResp gives an idea of the difference.   (The black line is open enclosure).

Field Coil Power Supplies

The Jensen A-12 Field Coils are powered by a full wave choke input tube rectifier supply. The field coils incorporate a Humbucker coil which I kept in circuit. (The signal to the VC goes thru a winding in the FC, so that any hum current in the main FC winding generates an opposing hum in the VC).

The LM555 field coils are powered by a Tungar full wave choke input supply, or alternatively 2 x 6V 110Ahr deep cycle batteries.  A relay in the battery circuit isolates the batteries when the system is not on.

Does the type of FC supply make a difference?  Is a Tungar rectifier really superior?  Does Selenium have anything over a sensible Schottky supply - or a cheap and easy 338 regulator? 

Field coil and voice coil share the same magnetic field so interaction between signal and power supply is quite possible (transformer coupling).

Differences in sound are subtle, as one might expect. Selenium sounded great to me but my rectifiers ran very hot - the fins need to be well cooled.  Schottky diodes also sounded just fine.  Both utilized the same hefty choke input circuit, perhaps this is the most important thing. 

I think there are differences, Tungar seems to highlight things  - 'they are here'.  The 6V 110AH batteries sound deep, spacious and clean - more 'you are there'. But then the batteries were providing a lower voltage so it could be the difference in field strength.

The WE555 FC specification of 6.3 - 7V is presumably for a Lead Acid battery supply.  The batteries would have been charged with a Tungar supply. Probably they left the charger on and kept the batteries on float.  The battery filtered the hum from the chargerů.  

"Holy Mackerel! " gasped Joe, the hardbitten WE service engineer." It sounds like a veil has been lifted!"  It must be the Tungar supply !!!

FC supplies are fun to play with but not a big thing to worry about, get started with a $25 338 regulator supply, above all it has to be quiet.

'Full Range' drivers.

'Full range' Lowther type cone drivers on a 160 or 204 horn have the speed and dynamics to make an alternative to the JBL2470 or 555.

Or make full use of them on the  Azura Push-Pull Horn.

 

Direct Radiator Bass plus Horn

Most diy horn projects are probably horns with bass enclosures, crossed over at  800 - 1000 Hz,  For this set up the choice of compression driver is limitless.

Lynn Olsen commissioned the AH-425 horns for his 'Beyond the Ariel' project on diyAudio. The horns were designed for mid format 1.4" throat drivers crossed over in the 800Hz region to a 15" driver.  The idea was to have a horn with similar frontal area to the 15" driver so that dispersion angle would be similar.

I now make a 1" throat version of the 425 horn - there are many 1" drivers that may be suitable.

Cross over design and attenuator is critical for this type of system. The Le Cleac'h 3rd order Butterworth method works well. This has a -5dB cross over point at the chosen frequency.

Set Low Pass f-3db to 0.87 x F
Set High Pass f-3dB to 1.14 x F
Move compression driver back 0.22 x wavelength F
Reverse polarity of compression driver

Try a RL final filter element (pole) in the 3rd order xo to incorporate the attenuation.

Dave Slagle of Intact Audio makes high quality autoformers to perform the attenuation of the mid upper horn.